A 62-year-old woman with a history of hypertension and hyperlipidemia presents to the hospital with diffuse muscle pain, weakness, and dark urine. She has no history of autoimmune or renal disease. Over the past month, she has had symptoms of cold intolerance, weight gain, and constipation. Her medications include hydrochlorothiazide, simvastatin, and gemfibrozil. Urine dipstick reveals 3+ blood; however, there are no red blood cells on microscopic analysis. Some of the other laboratory values are shown below:
Which of the following is the most likely cause of her presentation?
A. Hashimoto thyroiditisMedication effect. Muscle pain, weakness, and dark urine in the presence of an elevated creatine kinase (CK) should lead the reader to suspect rhabdomyolysis. This is further indicated by the blood on urine dipstick but no red blood cells on microscopy. Urine dipsticks bind hemoglobin and will be positive if there are red blood cells in the urine; however, myoglobin is similar in structure to hemoglobin and will also cause the dipstick to be positive in the absence of red blood cells on urine microscopy. CK is stored within muscle and therefore is increased in the setting of significant muscle cell destruction. Statins are known to cause myopathies, ranging from benign myalgia to severe rhabdomyolysis. Hypothyroidism is a risk factor for developing severe myopathies while on a statin, and the patient’s symptoms (weight gain, cold intolerance, and constipation) with an elevated TSH indicate that she has this risk factor. Another important risk factor for serious myopathies is the combination of a fibrate (e.g., gemfibrozil) and a statin. Initial treatment of rhabdomyolysis involves cessation of the offending agent (simvastatin) and correction of any fluid and electrolyte abnormalities.
(A) Hashimoto thyroiditis is the most common cause of hypothyroidism in developed countries, and may be the cause of this patient’s hypothyroidism. However, this is the risk factor for the patient developing rhabdomyolysis and not the cause. (B) Pyelonephritis is inflammation of the kidney and the urinalysis will be consistent with a urinary tract infection. This would not cause an elevation in CK. (C) Polymyositis is an autoimmune condition that can be diagnosed by the presence of anti-Jo antibodies. There is some overlap with the symptoms (muscle pain and weakness with elevated CK), but polymyositis would not present so acutely (symptoms develop more subacutely to chronically).